The long and the short of this is that there are several variables here, and methodologically it's really hard to pin down exactly what is happening.
1. AC quality coming into house varies.
2. Design of particular brand/model will have more or less hum.
3. Individual example of model will vary, according to quality of particular parts used to build it and the care with which it was built.
Although hum/no hum can be something of a predictor of performance of the piece of equipment in your hands in your house, it's never a guarantee that it will perform in the same way.